The present invention relates to a display device for colored displays and the like and more particularly to a light emitting display having an organic layer.
Recently, more and more researches on organic electroluminescence devices are made in view of their outstanding characteristics such as capability of being driven with low DC voltage, thin form, and self luminous property.
A conventional display device based on organic electroluminescence is shown in FIG. 6. The display device 1 includes a substrate 2, an anode 3, an organic layer 4, and a cathode 5, which are placed one over another. The anode 3 is a transparent electrode made of ITO (indium tin oxide). The organic layer 4 is composed of a hole injection layer 4a, a hole transporting layer 4b, and an emitting layer 4c to transport electrons, which are placed one over another. In such a device, electrons injected from the cathode and holes injected from the anode recombine in the emitting layer 4c, so that light radiates through the substrate 2. There is another type of display device which is composed of a substrate, a cathode 5, an organic layer 4, and an anode 3, which are placed one over another.
The above-mentioned display device has issues to be solved—extended life and improved reliability. The life of display devices is determined usually by initial deterioration (involving loss of luminance) and subsequent gradual deterioration. In other words, in order to extend the life of display devices, it is important to reduce the rate of initial deterioration and subsequent gradual deterioration.
Also, in order to improve the reliability of display devices, it is desirable to widen the region in which hole-electron recombination takes place so that excitons occur in a wide region. In an actual device, however, emissions are usually localized in the interface between the hole transporting layer and the emitting layer. This is one of the possible reasons for long-term deterioration. Consequently, it is considered that an effective way to reduce long-term deterioration is to protect the light-emitting material from local deterioration with time. In fact, it has been reported that green light-emitting devices greatly improve in reliability if the electron transporting emitting layer is doped with a hole transporting material. (See Applied Physics Letters, vol. 75, No. 2, pp. 172 to 174 {1999) and Applied Physics Letters, vol. 80, No. 5, pp. 725 to 727 (2002))
It has been reported that the device has a thermally stable hole transporting capability if the hole injection layer and hole transporting layer are made of tetraphenylbenzidine compound, triphenylamine trimer, or benzidine dimer. (See Japanese Patent Laid-open No. Hei 7-126615.) Examples of such organic materials include starburst amine skeleton represented by the structural formula (1) below and triphenylamine tetramer represented by the structural formula (2) below.
